The present invention relates to a solution for selectively stripping a layer of tin or tin-lead alloy from a substrate, which solution is particularly suitable for a spraying operation.
The necessity of providing a solution capable of selectively stripping a layer of tin or tin-lead alloy from a substrate, generally of copper, by means of a spraying operation, originates from an evolution of the printed circuits manufacturing technique. In the currently used technique the stripping of the alloy from the copper substrate is carried out only in the proximity of the connectors, whilst with the new technique of leveling in hot air it is necessary to carry out a complete stripping of the tin-lead alloy, whose only function is to provide during the etching stage a protective coating, from the entire copper surface. With such technique it would be highly desirable to carry out the stripping of the alloy by way of a spraying operation, inasmuch as the productivity would be a greatly increased and the necessity of manual operations would be reduced.
However, with the baths presently available in commerce, it is not possible to carry out this operation with the normal spraying machines, because these baths are highly corrosive in respect to the metal materials of which the machines are usually made, and therefore to use these baths it is necessary to convert the machines or to design new machines specially conceived for this specific purpose. In fact, the metallic components (recirculation pump, nozzles etc.) of the spraying machines available at present are generally made of titanium, a metal which resists very well to the ammoniacal ambient of the alkaline etchers normally used with such machines, but is quickly and strongly attached by the inorganic acids, such as chromic acid, nitric acid, and particularly fluoboric acid and its derivatives, as well as by other components, such as the peroxide solutions, which are contained in the formulations of the baths presently available in commerce for the stripping of layers of tin and tin-lead alloys.